Telephone trunking system.



- PATENTED MAR. 12, 1907.

I. W. DUNBAR. I TELEPHONE TRUNKING SYSTEM.

2 SHEETS-SHEET 1.

APPLICATION FILED AUG.1, 1902.

-PATENTED MAR. 12, 1907.

F. W. DUNBAR'. TELEPHONE TRUNKINGYSYSTEM.

' APPLICATION FILED AUG. 1, 1902.

2 SHEETS-SHEET Z 3 A ...U R B E R O F B W? GIMoM/wq.

June 2 ,1902, Serial No. 113,581, .a trunking systeinfiss'hown and described inwhich the UNITED STATES PATENT ,oFrIoE.

FRANCIS W. DUNBARQOF CHICAGO; ILLINOIs'ASSIGNOR TO KELLOGG SWITCHBOARD AND SUPPLY COMPANY, or CHICAGO, ILLINOIS, A

CORPORATION ILLINOIS.

TELEPHONE TRu mG SYSTEM;

State of Illinois, have invented a certain new and useful Improvement in Telephone Trunking Systems, e1 which the following is a speci fieation.

My invention relates to improvements in telephone trunking systems, and particularly'. those adaptedto be-employed in'connection with the s'o-called Uunbar or two-wire line-circuits inwhich thecu-t-ofi relays and other auxiliary apparatus are operat-ed .-ove r a portion of the talking-circuits,- wherebya great saving 1n expense and room results. v 4

In an application filed by William W. Dean,

trunk-relay is of high resistance, so that 'suffie cient current'cannot Ilowthrough it and over the metallic trunk-line to operate a super visoryirelayf in the calling co'rd circuit to rencl er the; associated supervisory signal inoperative. .l-In the present system-I contemplate accomplishing the samefresults by means of anordinary relay connected with one side of the; trunk-circuit at its incoming end and with'g'round, so that when a cord-circuit is I connected with the outgoing end oftlie trunk the current finds no path over the metallic circuit, and therefore the supervisory signal is not rendered inoperative until the .crlled metallic trunk-circuitare connected together I the preferable forrnthis trunk-relay is of low' circuit through the normal contacts of any other relay whereby a simplification in the subscriber responds and theltw'o sides 01 the to permit a flow ofcurrent therethrough. In

resistance and is included injthe metallic trunlccircuit so formed, whereby suflicient current flows to operate the supervisory 'relay in the. manner 'speci fied.

a My invention als'o contemplates :so arranging and so operating the trunk relay that it is unnecessary to carry t'he disconnect signalingapparatus results;

My invention further consists in the novelarrang'ement and combinations herelnaiter described, and particularly pointed out in the appended claims.

Patented March 12, 1907.

My invention is illustrated in the accompanying drawings, in .Which the same reference characters designate like parts through out the several views, and in which Figure 1 is a diagram showing subscribers lines leading to different central ofiices, to-

gether with the necessary trunking "conned tions between said ofiices to enable the sub s'cribers to be connected for conversation. Fig. 2 is a modificationfof the trunk,1and Fig.

3 is likewise a modification in the trunking connection.

the central'office ,C. This line extends-in two limbs-2 and 3 fromthe subscribers station A to the said central ofiice G, where it is fitted with suitable answering and multiplej acks or connection-terminals J'J At the subscribers Referring to Fig. 1, L designates one of the plurality of subscribers lines terminating at station-the usual telephone instruments-are provided and consists of a transmitter 4,,

.a receiver 5, a ringer or call-bell 6, and thecondenser 7. :The-switch-hook 8 is adapted in 'it'snor-Inal position and when the receiver is placed thereon to open the circuit through thetransmitter 4 and receiver 5 and to close it through the call-bell 6 and condenser 7.

A retardation-c0il9 is connected at the substationwhenthe hook is raised in parallel 'with the receiver 5 and condenser 7 to provide' a path transparent .to steady'currents, but-opaque to' voice-currents. The line conductors 2 and 3 include, respectively, the armatures-or springsIO and 1 1 of thecut-ofi relay R, which'is permanently legged to'ground from the-sleeve-contacts 7 1 of the 'jacks J 'J The armature or spring 10 is normally connected with ground through the conductor 12, while the opposite armature 11 is normally connected with conductor 13 containingthe jline-relayR for the line, said conductor leadi'ng'to the live or ungrounded pole of the central battery B, the opposite pole of whi ch'is grounded. The line-relay R c'ontrols' through its armature the circuit of the line-signal S. The forward contacts 14 and 15 of iZhQ'Cllt-Ofi relay R .areadapted to connect, the j acks and. the switchboard-section .of the line with the. external line-circuit when the said cut-oil relay is energized. This.'it is to be understood, is a typical two-wire linecircuit and is the particular kind with which I prefer to usemy trunk-circuits, although I do not wish to'so limit the use of the invention in all respects. 7

At the central office C the usual operators cord-circuit. is provided, said cord-circuit having an answering-plug P and a calling-plug, P each being provided with a tip-contact p. and a sleeve-contact 19*, adapted to register with like contact-surfaces and j in thejacks of. the line-when the plug is inserted therein. The tip-contacts of the plugs are connected together by means of the tip-strands t and t .of the cord-circuit and the interposed condenser'c, while thesleeve-contacts are like-= wise connected by the sleeve strands s and s and the interposed condenser A conductor 16 extends from the grounded pole of the battery-B to the tipstrand t and contains 4 the winding ofthe tip supervisory relay r associated with the answering-plug P, while the live pole of the battery is connected, by means of conductor 17, with the answering sleeve-strand sand includes the winding of the sleeve supervisory relay These relays ferred to as the tip-relays ca ling-plug P w The relays r and r are conveniently rer and 7 serve to control the circuit of the supervisory lamp S associated with the answering lug P, the former relay serving to normally 0 o s e said circuit of the lamp,-whi le the latterrelay normally opens the same.

The battery B? is associated with the op 0-] site orcalling end of the cord-circuit an is connected by conductors 18 and 19 with the tip'and sleeve strands t and 8 respect ely, the former conductor including the winding of the supervisory relay 1, while the latter conductor includes the similar relay 1". These relays serve in a manner similar to the relays r and 1' to control the circuit of the" su ervisory lamp S associated vrith the and relay r and 1: as the sleeve-relays, since they are connected with and operated by. currentflowing over the. corresponding strands of the-cord,

circuit. The tip-strand t? of the calling-plug and testing-plug? is normally open through the extraspring or movable contact 20 of the sleeve-relay r but is adapted to be closed by the said spring and-its forward contact 21;

' when the relay. is energized. The back contact 22 of this extra spring 20 is connected, through the medium of conductor 23, with one side of the high-resistance and high-im- 4 through the medium of conductor 26, to the pedance relay 1"", known as the test-relay,

live pole of the battery B a retardation-coil 27 beingdncluded in this circuit. The opera-j tors transmitter 28 is joined on one side to conductor 26 intermediate her -pirmary coil and the coiL27' and on the other side through conductor 29 to round. A condenser c is included in. the 'operators .local circuit to facilitate talking andis connected. as shown.

The secondary winding of her induction-coil.

25' is adapted to be included, together with her receiver 30 and a condenser c in a bridge .between'the strands of the cord-circuit, by

means of any suitable key-and indicated dia grammatically by k k. Branch connections 31 and 32 lead from opposite sides of the receiver and secondary to an order-key k connected with an order circuit or wire 33, lead:

A grounded alternating Y ing to the oflice C current ringing-generator g is adapted to be connected through a resistance-lamp with.

the tip-strand t of the cord-circuit by means of the. tip ringing-key spring W, the sleevecontact k of said key being with battery-lead 26 through wire 26 and resistance 26*.

A trunk-circuit is shown extending between central ofiices C and C, said trunkcircuit bein fitted atthe outgoing end with multiple 'jac (s, such as J and J and with a retardation-coil 34', connected to ground on one side and adapted to connect on the other with the sleeve side of the trunk-circuit when a connecting-plug is inserted in one of the jacks by means of the auxiliary sprin 34", which is pressed into contact with the s eevecontact of the jack when the tip-spring is lifted by the tip of the plug upon the inservtion of the same lnto the jack; The truq kcircuit extends in two limbs 35 and 36 to the central office C where it isprovided with suitable means for testing the condition of the wanted line, for ringing the desired subscriber, and with signals to enablethe operator to properly attend the connection. The tip and sleeve trunk-conductors 35. and 36 terminate at the incoming ofiice in the Windings-w and w of the repeating-coil W, the winding to being connected by conductor 36 with one side of the low resistancetrunkrelay 1", the other terminal of w ich is connected by conductor 36 with t earmature '37 of the tip-relay 1*", said armature being normally grounded through its back contact and being adaptedthrough its forward contact and the conductor 35 to complete the .metallic circuit of the outgoing end of the trunk when the. tip-relay is actuated. A

condenser c is connected in parallel with the trunk-relay-r to furnish a free passage for the voice-currents passed by the trunk-relay, The opposite windings 'w and. 'w' of the repeating-coil W are connected with an interposed condenser 0 between the'tip-strand t and the sleeve-strand s of the flexible end of the trunk-circuit, which strands terminate in the corresponding contacts 10 and p of the trunk connecting-plug .1 A tip-relay r is legged to ground'from the tip-strand t of the trunk-circuit and controls, through its" spring A sleeve-relay r is connected on one side with-the live pole of the battery B? by means of conductors 40 and 41 and on the other side with the sleevestrand' s This incoming endof the trunkcircuit is provided with a testing ap aratus similar to'that described for the .cor -ci rcuit at .the central ofiice C. The forward.- portion of the tip-strand if is connected with spring.

4 2 of the sleeve-relay 1"", while the other portion is connectedwith the forward contact of tending from the point 54 to the front contact said spring. I The back contactof spring 42 l is joined by conductor 43with the common high resistance and impedance test-relay 1- the. opposite terminal of which is grounded. The armature of this test-relay serves when actuated to ground one side of the primary winding of the operators induction-coil 44, which winding is connected on the other side with a conductor'45, containingthe resistance and impedance coil 46 and joined to conductor 47, leading directly to the live pole of the battery B The operators transmitter 48 is in a conductor 49, leadingfrom con-' ductor 45 to ground. A condenser c is connected between conductor49 and the side of the'primary that is adapted to be grounded. The operators receiver 50, together with the secondary of her induction-coil and condenser c7,-are connected permanently with the or'der circuit or wire 33, leadin to the central office-C. The conductor 40 leads to the point 54, from which point branch 55,

containing the ringing-lamps, leads to the back contact-of the armature of the lockingrelay 1', said armature being connected by conductor to the point 52 u on the conductor 52, extending between t e spring. 57 .40-

of tip-relay r and the front contact of s ring 51 of sleeve-relay r, the latter spring eing' grounded, as shown. A conductor 56 exofsprin'g 57 of tip-relay r ,'inc1udes the wind- The'forward of conductor 58. The .back contact of:v the trunk-relay r is connected by'conductor-59 The arrnature' of they with alcontact '52..

' trunk-relay r is; connected. with'a conductor 53, leading to the 'battery wire 47 and in cluding the guard and disconnect lamp and the pilot-relay -r ,said latter relay ser ing to control the circuit of the pilot-signals. 3 ,Thisincoming end of the'trunk is equipped for selectively ringing subscribers upon a party-line. It com rises the ringing-key contacts 7c 7; and k placed the strands of the trunk-cord and adapted whenactuated to suitably connect the alternating-current generator'g with the plug P This rmging-generator comprises the' armature 60, the-grounded brush 61, the brush 62 from which the alternating current is directed key k -when operated connects (the sleeve of the plug to battery-lead 47 through resistance-coil 66 to maintain the cut-off relay of the line operated during ringing. The subscribers line shownin connection with this office is of the party-line type-and is connected to subscribers D andD". The apparatus at these stationsis similar tothat at the station A and are likewise indicated, except that the bell .6 at station -D is connected be- 'tween'the sleeve conductor 3 and ground,

while at station D the bell is connected between the tip-conductor 2 and ground. The

same type of, normally disconnected jacks J J line-relay'R cut-off relay R, andthc' line signals S are employed and are designated by the same reference characters. The battery B furnishes current for the operation of these various relaysand signals, as well as for talk- $1 J and J J 6 are respectively busybac and dont-answer jacks associated with the incoming trunks. These circuits comprise an induction-coil 67, having its primary in circuit with a rapidly-rotating cir' cuit-breaker 68 and the battery B, this battery being preferably the same as battery B 'terrupter.70. From interrupter 69 a con-' ductor 71 leads to the tip-contacts of busyback jacks J J through a fifty-ohm resistance-coil 72,.while interrupter 70 leads by conductor73 through-resistance-coil 74 to The the tips of the dont-answer jacks.

sleeves of both sets of jacks are united by conductor 75 with the grounded pole of the battery. The. circuit-breaker 68 causes induced currents in the secondary circuits which are interrupted at short intervals by breaker 69 pro and at longerintervals bybreaker 70 to cause distinctive tones at the j'acks J 5 and J f, where- I by when a'trunk-plug is inserted in one or the other jack a corresponding signal is transmitted to the waiting subscriber.

In tracing the circuits hereinafter the plan for pointing out' the path for current from the live or ungro'unded pole of the battery to the ground oniy will be followed for convenience, it being understood that the return will seek 1 the grounded pole of the battery and that the circuit will therefore be complete.

The operation is as follows: The, A subscriber desiring a connection with a subscriber located at another exchange removes his receiver from the hook, and thereby closes a path. for current between the limbs I Z;ductor 26 through retardation-@1127, thence 2 and 3 of his line-through the transmitter 4 and retardation-coil 9, the condenser 7 and receiver 5 being connected'in parallel-with the retardation-coil. The closing of this circuit permits current to flow from the, battery B through conductor 13, linerelay R spring 11 of cut-off relay limb. 3 of, the telephone-line, through retardation-coil 9 and ,transmitter4 at the substation, and

thence over limb 2 back to the central 'oflice- Y and spring 10 of'cut-ofi relay R, through conductor 12 to ground. The line-relay R is thus operated and closes the circuit of the signal-lamp to ground from the conductor 13. The operator-upon'seeing the line-signal exposed inserts the answering-plug into 1 the answering-jack J of .the subscribers line andcon'nects'her telephone with the cord circuit by depressing the listening-key. k k to receive the-order from the subscriber. The insertion of the plug P closes a circuit through the 'cutmfi relay Rfrom the live pole of-the '1 battery '13, through conductor 17, sleeve supervisory relay W, sleeve-strand s, sleevecontacts p and 1 of the plug and jack, thence through the winding of the cut-off relay R to ground. The operation of this relay disconmally disconnected-jacks J J nects the armature 10 and 11 fromthe the conductor 16 to the grounded pole of the battery B as soon as the cut-oil relay R is operated. The supervisory signal S there- 'fore remains inert while the battery B fur-..

nishes current over the metallic telephone line for the operation of the supervisory relays and for conversational purposes. The operators transmitter 28' is receiving current at this time from the battery B 'over the conthrough the transmitter28 and over conductor 29 to ground. When sound-waves strike the transmitter 28, the current flowing therethrough is varied, which causes a variation oi the charge in the condenser c. and a corresponding surge of currentthrough the primary of her induction-coil; 25. These induce voice-currents in the secondary, which are transmitted to the'line, and the operator is therefore able to converse with the calling subscriber. Upon learning that a subscriber in the exchange C is desired, the A operator depresses the order-key k to connect her telephone with the order wire or circuit'33 and repeats the number of the wanted subscriber to the B operator, whose head-telephone is permanently connected, with the said order-wire. The latter operator designates to the A operator the trunk to be used 'for the connection and proceeds to test the condition of the coming trunk named. 1 lithe line is idle,'no click will be received, but if busy, the

sleeve-contacts of the i1acks of theiline will be connected with the ive pole of a battery --wanted line with the tip of the plugof the inthrough the'sleeve contact and strand of the inserted plug. A complete circuit will there- 'fore be established from the-sleeve-contact of the tested jack, which, it is assumed, is

connected with the live pole of the battery, through the tip of the plug 1-, tipstrand t spring 42, and back contact of sleeve-relay r conductor 43,- through the common high resistance andimpedance test-relay or respon sive device r to ground. This test-relay will thereforeoperate and ground through its armature and front" contact one side of the primary of the operators induction-coil 44, the other side of which is connected by conductor 45 through resistance 46 and conductor 47 to the'live pole of the battery B The completion of this circuit will permit a flow of eurrent'therethrough which will induce a current in the secondary of the saidinduc- 'tion-coil and produce a click in the receiver 50 of the operatorsset. The operators receiver Will-respond to this click at all times,

since the-electrostatic capacity of her circuits and that of the order-circuit is ample for-the purpose of producing the click.

In the three-wire systems where the testcircuits are insulated from the suhscrihers line there is no danger of varying the current .fiowing in the line to the annoyance of the subscriber during testing; but in the two; v wjresys'tems',,whcre the'testingycontacts are connected directly with the talking-circuit,

some means is necessary or at least desirable to prevent noticeably varying the current upon llnes connected for conversation when the circuit described bymeans of thetest-re- IIO testing their'c'ondition at other sections of the switchboard. This is accomplished in,

lay r whichis of such high resistance as to slightly and which is also of high impedance, so .that its' current will build up slowly, to

thereby prevent a sudden variation of cur-' rent on the line. The subscriber or'subscribers Whose lines are tested are therefore undisturbed by a sudden or largevariation of current through their instruments. It is apparent that any other suitable test responsive device or indicator than the relay 10 can ,be employed and that the-operators' telephone can be used in place of the same oriin the circuit with a retardatiomcoil'; but the arrangement described iis preferred; This .vary the current upon the tested line but relay is preferablycommon to all the incom 7 ing trunks at one operators position, as indicated' by the branching lines at 43 Upon finding the line idle the plug P is inserted in one of-theiinultiple'j-aclrs of the wanted sub- 'Bio'perator rings the wanted subscriber. If

scribersline; I

The A-o'perator hav ng received the number of theplug to be used'inserts the plug 1 of her cord-circuit in the jack J 3 of the trunkrc at her section. This'connects the retardation-coil 34 with the sleeve side of the circuit and provides a directpath for-current from .the battery .13? over conductor '19, 'stra'nd" s sleeve-contact of plug'and jack'to the retardation-coil134 toground, thus positively operating the sleeve'supervisoryrelay 11 which attracts its armatures but cuts out the test relay 7".- and completes the tip-strand of the -cord 'cir'cuit through armature and its for- Ward cont'act'Zl. At the same time'a. path for current from the battery B is provided ove'r the sleeve-strand ofthe cord-circuit and the sleeve conductor 36 of the trunk-line through winding 21) of the'repeating-coil W,

conductor 36?, trunk-relay 11 conductor 36 armature 37 and back contact of relay 1? to ground; This relay is therefore operated and opens the circuitiofthe disconnecting'-- lamp S and 'pilot'-relay 11 The supervisory signal .8 connected with the A cordcircuit -is also operated, for thefreasonhthat current is entirelycut off froni't h e tip relay r. The A operator is therefore-informed :that the called subscriber l'ias not yet responded. By the 3 5 insertion Ofthe incoming trunk plug in the ofthe called line and thencethr'ough the cut off relay It to ground; The sleeve-relay and the cut off'rel ay. are thus' operated,the latter serving to disconnect the linefrelay-R to render the line-signalinoperativeand to connect the normally disconnected-- jacks with the limbs of the line iexten'cl'ingrtozthe substations. The operationxof the sleevere- 50 the tip-strand through "the spring 42 and its forward contact, and thereby establishes the circuit for conversation. It also cornpletes the circuit of the ringing-lamp Stfro'rn" the battery B over conductor to the: point 54.,t1icnce, over conductor and;

through lamp 8* to' thebackcontact and. armature of the lockingrelay 1 thence through the 1 armature and overiconductor Ward contact of spring 51 ofthe sleeve-relay r and thence to ground. .The completion of this circuit causes'the operation of the ring .in'g-lai'np S which indicates tothe operator that the subscriber has not yet responded.

fsubscribers b'ellover the following therefore completing the circuit between the condenser 'Z- and bell.6'.

jack of the wanted'li'ne a circu t is closed. from the live pole of-the-battery. -B over con,- ductoIS-i 40 and 141', through sleeve-relay rtand thence from the sleeve strand is gof -the H flexible end ofthe trunk cord-circuit through 5' vide a path to ground for. the return current lay r disconnects the testrelay r and closes- 55 to point 52 over conductor 52' to the for- The operation of the trunk-relay r at this time opens, through its armature and back the subscriber 1) is the one desired, the ring ing-lgeyz k k is depressed, which ,throws ringing cu'rrent upon the sleeve-strand connected with the plug P and-operates the ath: from ground at the generator 9", brus 61, armature 60 of the generator, brush 64, through resistance-lamp 65-, key-k thence over the sleeve-strand of thecord to the plug-8o P over the. sleeve-limb 3 of the telephone- .line to the station D, and thence through condenser 7 and bell 6 to ground,,the receiver 5 being;at=.this time upon the hook 8, and

will be noticedthat just previous to the de ression of theringingkey k the. cut-off re ay R was receivinga negative current throughf-its 'co-ils from thebattery B At the instant 'therin ing-key k is depressed this path of curren from the battery B3. is broken, but anew path is immediately established through-the relay R from the. negative source of pulsating ringing-current; The

relay Bis sobon'structed as to be maintairied in. continuous operation when this negative pulsating current is passing through its windings, and as ulsations .are used of the same-direction ast e current. furnished by the battery B the relay R is prevented from even fmomentarily. releasing its armatures at l the .Inoinent the ringing-key was depressed. "The bell 6 at the subscribers station may be loifthe ordinary 'po'larizedtype, as the pres- .ence of the' condenser 7, ,Which is connected directly in the ringing-circuit. permits the operation ofsuch'a bell from the pulsating source of ringing-current. Thetip-sprin'g off if the subscriber should respond during ringing. Should the subscriber. D? be the one desired, the ringing key k k is depressed,

which serves to connect the generator" 9 15 with'the tip strand .of'the cord-circuit, to op-- erate the bell jat'the-statiQnD The ringing-ci rcuit may be traced as follows: from ground' at the generator 9, brush .61, armature-winding-60 of'the generator, brush 62,

through resistance lamp '63, spring 10' in the tip-strand of the; cord-cricuit, thence over I the tip-strand and tip-conductor 2 of the line to the. station "D and thence through condenser. 7 and normallyclosed contactsof the hook 8 and through bell 6 to ground. The

bell is thus operated. To maintain the cutoff relay-R operated during ringing, the in her contact of the sleeve-spring k is connected through a resistance-coi166 with the battery lead 47, extending to the live pole or the battery B This prevents the .denergization of cut-ofi relay it while ringing. It is apparent that the arrangementof the ringing-keys is such that either key may be used to call a subscriber at whose station the apparatus is connected in the manner shown at A, which shows the arrangement which I preferably use where but one subscriber is placed upon the line. In this case theringking-circuit would be complete over the submitter 4 at the substation, thence over limb 2 of the telephone-line back to the central office and over tip-strand t of the incoming end of the trunk, including spring 42 and its forward contact of sleeve-relay 1', through conductor 36 and tip-relay 1' to ground.

This relay thus responds and opens the grounded circuitothrough the'trunk-relay r by means of its spring 37 and backcontact and closes a circuit therefor through its forward contact and through conductor 35,

the repeating-coil winding .w to the tip-conductor 35 of the trunk, the tip-strand of the cord-circuit and conductor 18 to the op osite pole of the battery B The trunk-re ay 1" and the tip supervisory relay 1" are now included in the metallic circuit; but the resistance of the trunk-relay being small suflicient current flows therein to operate both, where- 'by the disconnecting-lamp and the pilot-relay at the B exchange will be prevented from operating as before, and the circuit of the supervisory signal S is broken, so that the said signal is rendered inert, thus indicating to the A-operator that the subscriber has responded. The closing of spring ,57' of tip-relay r upon its forward contact completes the circuit of the locking-relay r from the live pole of the battery B over conductors 40 and 56 and through armature 57 to ground. This relay therefore operates and cuts-off the ground connection from one side of the lamp 8*, which becomes inert and closessaid ground connection upon its forward contact, which completes a second path for current through the winding of the relay 1' which path is'in dependentof the tip-relay, but, as before explained, is completed through the spring 51 of the sleeve-relay. Should thetip and sleeve contacts of the plug be crossed when the plug is inserted in the jack and the look- I ing and other relays operated, they will be immediately released by the opening of the gether for conversation, and the supervisory signals at both exchanges are extinguished.

The battery B is furnishing current over the answering end of the A cord-circuit to the A subscriber's line for talking purposes, and

for the operation of the supervisory relays the battery. B isfurnishing current over the trunk-line for the operation of the supervis'ory relays associated with the answering end of the A cord-circuit, and the battery B 1s sending current over the incoming end of the trunk-and to the substation D to furnishcurrent for talking, and for the operation of the several relays. The voicecurrents,are transmitted inductively from the subscriber's line L to the trunk-circuitthroughthe condenser g: andc in the A cord-circuit and from the trunk-circuit to the incoming end thereof and the subscribers line connected therewith through the medium of-the repeatingcoil W. The condensers betweenthe windings w? and w of the repeatingecoil at the-incomingend of the trunkprevents the flow of I steady current therethrough, but permits a] free passage of the voice-currents.

At the terminationof the conversation the subscribers return their receivers to their hooks with the result in the case of subscriber A that the tip supervisory relay 1",

connected with A C()I'd'-C1IG1llt,iS deenergized and closes, through its armature and backcontact, the circuit of the supervisory signal 8?, thus indicating to the operator that subscriber A has returned his receiver to the hook. When the subscriber D hangs up his telephone, current is cut off from the metallic line in the same way, and thetip-rel ay r is therefore denergized. This serves to open the metallic circuit of the outgoing end of the trunk, thus deenergizing' the supervisory relay r in the A cord-circuit, which closes the circuit of the superyisory'signal S and indi-.

cates to the operator that the D subscriber has hungup his receiver. The trunk-relay 1' however, is again operated over the grounded circuit, asdescribed. The lockingrelay remains operated over this ringing-circuit to prevent the operation of the ringinglamp S. The A operator seeing both signa s in her cord-circuit exposed pulls out b th plugs, thus restoring all parts to normal at her osition. The withdrawal of the plug 1? at t e A exchange deprives the trunk-relay 1' at the incoming end of the trunk of op eratingcurrent, which is therefore denergized and closes, through its armature and back contact, a circuit of the disconnecting-lamp S and of the pilot-relay 1*", said circuit being traced from the live pole of the battery B over conductor 47, through the pilot-relay 1' and the lamp S over conductor 53 to the ars. .r3c

mature of the trunk-relayn, thence over its tion of-trunk'relay r, and therefore preventmg the operation of. the disconnecting-lamp.

.the trunk circuit, if preferred,

back contact, the conductor 59", conductor 52,forward contact and armature 51 ofsleeverelay 1" to groundi The pilot-signal S is therefore operatedto attract the attention of the "operator'to'the bankof incoming trunk disconnect-signals, and the lamp S .indicates the; particular trunk to be disconnected. The B operator therefore withdraws the .plug 1?; thus rendering the signal-lamp S inert, and denerglzing pilot-relay r and locking-relay r All parts are thus returned .to normal position. a 1 f Upon the withdrawal of the'p'lug P as ex pla1ned-, 1"'rom the-.trunk-jack the retardationcoil 34 is disconnected therefrom, thus pre-" venting the operation of thet-runk-relay 1" by; means of theso-called earth-currents,whicl1;

may be present in sufiicient amount to flow over the grounded side of the trunk-circuit, it the retardation-coil is. permanently connected, and thus 'preventmgthe .den'ergiza- It will be understood, however} that the re tarda tiomooil may be connected permanently with' the sleeveside of the outgoing-end of f The d 'connectingsignal S5 serves also as a calling-plug P of'he'r circuit into the wrong jack, for the reason-that the trunk-relay 01' that trunk will be operated, whil e the sleeverelayis not operated. A circuit will thereforebe completed from the live pole of the battery 'B over conductor 47, the pilot-relay 1'" and disconnecting-lamp S conducitor'53, armature, and forward contact of trunk relay,-conductor 52',- back contact, and spring 5lof sleeve-relay?" to ground.

This lamp willtherefore glow and indicateto 4 the operator that a mistake has been made.

In case the wanted subscriber's [line is iound busy or in case"the sub'scriber having been called does not respond,;-theplug I? is.

inserted-in one of the jacks J or"J.",-"aiid the corresponding signal istransmitted to the Waitingsubscriber." 1

. Upon inserting the plug 'Ptin one of the busy-back or dont-answer jackskfor 8X.- ample, one of the jacks J a path "for c,ur-

rent is provided from the battery B through' the sleeve-strand of the' trunk-cord, sleeve:

contacts of the plug and jack, and over 0011- ductor to ground. This will serve to op-- crate. the sleeve-relay r to complete the continuity of the tip-strand t of the trunkcord.

through its'spring 42 and the front contact.

Upon the completion of the strand at this" .poi'nt'the tip-relay 1' is provided with current fro'm the ,live pole of the generator 13, through the secondary winding of .the inductron-coil 67,-throughcircuit-breaker 69, and

. thence over conductor 71;,thr0ug-h resistance 72 and the tip-contacts-of the j aek-an-dplug,

and over the tip-strand, and through the re-,

guard-lamp in case the'A; operator-inserts-ithe lay r to ground. This circuit-is-interrupted at the breaker 69, so that there'lay r is alternately energi-zed and decnergizqed to corre spondi'ngly make and break the short circuit of the trunk-relay 7*, which is therefore 'si y the circuit of the outgoing end of the trunk. This causes the intermittent operation of the tip-relay If in the A cord-circuit to effect the flashing o'fthe supervisory signal S, whereby theA' operator is notified that the called-sub- .scrjihers line is busy. The flashing of this light of course corresponds.to 'the rapidityof Although 1 ha described: the method of making. connections, through the medium of the order-circuit, I not wish to's'o confine "the invention-,-*-f or' itjis apparentthat the'A 'smultaneously removed from and inserted in operator is able by means ofihjer testing apparatus to. test the condition. of tjlie" Several trunkSbefore her until. an idle trunk is found. Upon so testingiand finding an idle trunk she may insert'the calling-plug P of'her cordcirc u'it witl'rtl e-resu'l't that the trunk-relayr operatestand lights the lamp S which lamp has-been previously termed-the -guardland 15- but in'this method of: operation 1t won (1 act as a calling-signal from' the A op- 'erator to the B operator. 'The'iB operator, ju'p'on seeing-the signal S would depress her listening-key 16 k which would connect her receiver in series with the repeating-coil windings w and Z04, these receiver connectrons being indicated in dotted'lmes 1n Flg. 1. She,1s therefore I able to communicate with the Aoperatorf Itwill be understood also that had thesubscriber A called for a connection with another subscriber in the same eX- change the line of that subscriber would have been tested-in the ordinary manner by means of the, cordplug P a ndthev 'su'bscriber' would have been-called by the ring ing-generator g. i

Fig. 2 shows a modification of my inven tion. in' whioh the sleeve conduotor'of the trunk at the'B' exchange passesby means of conductor 36 to the armature37'of the tip- 'relay W, the back contact-of-"which -is connected with a'oonductor 36.; leading to ground through the trunk-relay 1", this relay being also of low resistance. The forward 3 3 q ductor 35 of the trunk, while thesimilar concontact of spring 37 of tip-relay r connects,

through conductor 35, with-the repeatingcoil winding w" and the tip conductor -35 of the trunk. When, therefore, the tip-relay operates, the branch containing the trunl relay is entirely disconnected from the sleeve conductor of the trunk and the two. conduc .tors of the trunk are connected together through the armature 37 and its forward contact. In view of this arrangement, in which the trunk-relay is'deenergized during conversation and during the period of operation of the tip-relay T the circuitzof the disconnect ing-lamp, is extended in series through the back contact and spring-57 of tip-relay 1 v In order' to operate the disconnecting-lamp, therefore, the simultaneous deenergization of both thetip-relay and the trunk-relay is re.

-quired; otherwise, .the construction and op- The retardation coil is here shown connected, permaeration is the same as in Fig. 1.

. nently with the sleeve-strand at the outgoing end ofthe trunk, but it will be und: :-stood liihat it maybe normally disconnected,as in 1g. 1. In Fig. 3 a slightly-different arrangement is presented, in that condensers are used in the strands of the .trunk in place of the reeating-coil W. The condenser 0 is placed etween the tip-strand t and the tip-condenser c is included between the sleevestrand 8 and the conductor 36 of the trunk; otherwise, the operation and construction is the same as in Fig. 1, and is therefore not specifically described. Itwill be apparent that the voice-currents will be transmitted from end to end of the trunk through the in ductive action of the condensers, while the steady currents will be forced to take the paths corresponding to those heretofore described. In this figure also I have omitted the retardation-coil at the outgoing end of the trunk and have provided a make before brea contact for the tip-relay 1, ,whereby when the cord-circuit is connected with the trunk at the outgoing end the sleeve super- I visory relay is actuated over one side of'the trunk and through the grounded contact of the tip-relay, and when the called subscriber responds the metallic circuit of the trunk is completed before the ground is disconnected, to thereby prevent the dee-nergization-of the sleeve supervisory relayin the A cord; It

will be understood that the return-circuit over which the trunk-relay i s actuated may bev a common return-wire instead of the ground-return, as heretofore described.

The ground connections heretofore referred to, it will be understood, may, and in practice usually are, one and the same, or they may be the common oflice-return, although it is found "by experience that it is practically necessary to place a dead ground upon'one side of the common battery. It is also apparent that the other poles of the bat teries may be grounded so far as the operation of the system is concerned, the only varia-1 tion' necessary from that shown inthe dra ing being to remove the present"; ound cIjo nections and replace them byoter'conduc- I tors and replace the ground upon-theoppd site pole of the battery. While the exact dimensions of the relays are not the essence of my invention, I have secured good results with trunkrel'ay's from two hundred and fifty to five hundred ohms resistance, tip and sleeve-relays of onehundred ohms resistance,

and loclring-relays of five hundred ohms,

While I have described several forms'and I several different ways of carryingout my 111 vention, I would have it understood that the invention is not so limited, as it is appa'rent that various changes and substitutions may be made therein andstill come .within its scope and principle; but

What I do claim as my invention, and de- -sire to secure by Letters Patent, is I 1. The combination with a trunk-line adapted to connect with a calling telephoneline at its outgoing end and a called tele- 'phone-line at its incoming end, of a guardsignal'at the incoming. end of the trunk means operated by current flowing over one side only of the trunk for placing said signal in condition to operate when the connection is established at the outgoing end of the trunk; and further means actuated by current sent over a portion of the talking-circuit when the trunk is connected with they called line and before the response of the called subscriber for rendering said signal'inoperative, substantially as described.

2. '.The' combination with a trunkline adapted to connect with a calling telephone: line at its outgoing end and with a called telephone-line at its incoming end, of a' guard-signal for theincoming end of the trunk, a relay actuated over one side of the trunk and ground when the connection is established at the outgoing end of the trunk for operating said signal and a second relay adapted to be actuated over one side of the talking-circuit when the connection with the called line ise'stablished, said latter'relayserving to prevent the operation of said signal, substantially as described.

3.' The combination with a trunk-line adapted to be connected with a calling telephone-line at its outgoing end and a called telephone-line at its incoming end, of -a signal associated with said incoming end, 'a'relay also at said incoming end adapted to be operated bycurrent sent over one side of the trunk and ground before the Subscribers are in conversation-and over the metallic trunk line during conversation, and'a second re'lay at said incoming end adapted to be erated by current flowing over a portion of the talk: ing-circuit whenthe'trunkis connected with the called line, said relays jointly controlling said signal so as to operate'the same to give 'LO a guard. indication when the connection is established at the outgoing end only and so.

as to give a disconnectindication when theconnection is severed at the outgoing end and the incoming end is oonnected with the called line, substantially as' described.

3 x 4; The combination witha' trunk-line adapted to beconnected with a calling tele-.

phone-line at its outgoing end, and with a called telephone-line at its incomingend, of a relay actuated over one side of the. trunk and ground before the subscribers are in conversationand over the metallic trunk during conversation, a second relay adapted tobe 0 erated over one side of the talking-circuit w en the trunk is connected withthe called 'line, and a signal forthe incoming end of the 'Jtrunk, said signal-rbeing-actuated when either incoming end-oi the trunk'responsive to current over one side only of the trunk' with suitable return whenthe connection is established atthe'outgoingendand the called sub-1' scribers telephone is 'not'in 'use,"a second relay at said end actuated over a portionof the talking-circuit when connection is estab- V lished with'the' called line, a signal whose acnal also associated with the trunkand ac'tu-f ated whenever said first-.named signal is ac- Y over the-other side of 'the talkin -circuit when the called subscriber respon s, sub -stantially as 'described.,' v

tuation depends upon said relays and adapted'tobe operated to indicate when a connec-..'

tion exists at either end only, and apilot-sigtuated, substantially as, described.-

6. The combinationwith a trunk-line 63F tending between different switchboairdsections and adapted to connect telephone-lines together for conversation, of a relay atIt'he incoming end of-the'trunk responsive to cur rent 'overvone-side only of the trunk with suit-- able return when the connection is j'e'stabi lished at the outgoing end and the called 'subscribers telephone is not in use, a second re- 4 a lay atfsaid: end actuated over'a portionof the talking-,ircuit when connection "is estab-- lished with the called line, a signaliwllose actuation-depends solely upon said relaysand adapted to be operated to indicate when a connection exists at'either end only, and a visory signal inoperative described.

operative condition by current .over one. side of the talking-circuit an through the retardation-coil, said signal bepilot-signal also associated with the trunk and actuated Whenever said first-namedsig nal is actuated, substantially as described.

7. The combination with a trunk-line extending between different switchboard-sections and adapted to connect telephone-lines togetherfor conversation, of a relay at the ncoming end of thetrunk responsive to current over one side onlyof the trunk with suitable return whenv the connection is established at the-outgoing end and the called subscribers telephone is not in'use, a second reat said endactuated over a portion of the talking-circuit when connection is established with the called line, and a signal entirely controlled by said relays and actuated when connection exists at either end onlywherebyit acts under one condition as a guard-signal'and-under the other as a discon nect-si nal, substantially as described.

8. T e combination with a trunk-line, ofa cord-circuit to connect with its outgoing end and a calledsubscribers line with which its incoming end is-adapted to be connected, of a retardatlon-coil legged ,to ground from one side of theitalking-circuit established by the connection of the cord with the trunk but normally disconnected therefrom, a super- 1 visor-y signalassociated with the cord-circuit and placed in condition to operate by 'current flowing throu h said'retardation-coil and means actuate by/the response of the called subscriber for rendering said supersubstantially as 9. The combinationwith a-trunlc-line, of a cord-circuit to connect with its outgoing end of a called subscribers line with which its incoming end isto connect, a retardation-coil legged to ground from oneside of the talkingcircuit established by the connection of the cord with the trunk, but normally disconnected therefrom, and 'asupervisory signal associated With the cord-circuitand laced ing rendered inoperative by current flowing 10. The combinationwith a trunk-line, of

Ra; cord-circuit to' connect with its outgoing end and s called subscriber's line with which its. incoming end is to connect, a retardation-coil legged to ground from'one side of the talking-circuitflestablished by the con-' nection of the cord with the trunk, saidicoil being normally disconnected therefrom, a

supervisory s'ignal -in'iconnection with the. cord and placed inoperative condition by owin other side' of the cord-circuit upon the response of the called subscriber to render said.

' signal inoperative, substantially as described.

11. The combination with a trunk-line, of a cord-circuit to connectiwith its outgoing end and a called subscribers linewith which its incoming end is to connect, a retardationcoil legged to ground from one side of the talking-circuit establishedby the connection of the cord with the trunk, said retardationcoil being normally disconnected-therefrom,

arelay actuated by current flowing over one i side of the cord-circuit and through said retardation-coil, a supervisory signal associated with the cord-circuit and'plac'ed in'operative condition by said relay when actuated, and a second relay actuated by current flowing over the other side of the talkingcircuit. upon the response of.the called subscriber to render said signal inoperative, substantially as described.

12. The, combination with a trunk-line, of a cord-circuit to connect with its outgoin end and a called subscribers' linewith whic its incoming end is'adapted to connect, a retardation-coil legged to ground from onezs'ide. of the trunk-circuit during conversation only, said retardation-coil being normaliy disco n' nected from saidtrunk-circuit, a superyisory signal associated the cordecir'cuit and placed in operative conditionby current flowing throug said reta'r'dation-coil, and means actuated upon the response ofthe called subscriber for rendering said supervisory signal inoperative, substantially as described. "ThebombinationWith a trunk-line, of a cordcircuit' toconnect with its outgoing endanda called subscribers linewith which its incoming end is to connect, a retardationfrom the trunk, a. supervise I ated with the cord-circuit an placed in con'' coil legged to ground from one side of the trunk-circuit during'iconversation'only, said retardation-coil being normally disconnected signal associdition to operate by currentoverone-side of the talking-circuitand through said retardae tion-coil to ground, and means actuated'by current flowing over the other side of the trunkcircuit upon theresponse of the called subscriber for rendering said signal inoperative, substantially as described."

14.: Thecombina'tionwith a trunk-line, of

a cord-circuit to connect with its outgoin end and a called subscri'b'ers line with-whic itsincomin coil normafi of the called subscriber for rendering said supervisory signal inoperative, substantially as described. I

tiOn-cOiI- bein "the circuit w, ereb end is to connect, a.retardation-' y disconnected vfrom the trunkcircuit but connected with one side .thereof .by the connection of the cord-circuit with the trunk, a supervisorysignal associated with the .cord and placed in o. erative condition by current flowing throu said retarda'ticncoil, and means actuate upon the response when the cord-circuit plug is inserted in the jack, 'a superyisory signal associated with the cord-circuit and placed in operative condi- 'tion by current flowing through said retardation-coil, and meansactuated by the response of the called subscriber for rendering said signal inoperative, substantially as de-' scribed.

16.. The combination with a trunk-line adapted to be connected with a calling telephone-line at its outgoing end and a called telephone-line at its incoming end, of an elec- Itrom etic device normally' legged to .rgroun at it's incoming end for'theoperation of "certain telephonic apparatus, and .a retardation-coil legged to ground at the outgoend-during. conversation over which current is adapted 'to'flow for'the operation of certain telephonic'app'aratus, said retardathe circuit w ereby the .presence of earth currents cannot normally cause the undesired operatlon of the said electromagnetic device, substantially as described.

17. The combination with a normally disconnected from trunkdine adapted to be' connected with a calling telephone-line at its outgoing end and a called telephone-line at its incoming end, of a relay I normally legged to ground from, one side of the trunk-circuit at ltsincoming end andar ranged to control telephonic apparatus at said end, and a' retardation-coil legged to ground a't'the outgoing-end during connec t-ion over which current is adapted to flow to operate telephone apparatus, said retardatron-coil bein normally disconnected from currents will not e ect the undesired operation of the relay, substantially as described. -18. The combination with 'a trunk-line adapted to be ponnec'ted with a calling tele-.

phonedine at: Its outgoing end and a called .telephone-line' at its'incoming end, of a relay i normally legged to ground-ffrorn one" side of the trunk-line at its incoming end for the op-. eration of'telephonic apparatus, and a retardation-coil legged to ground from the same side of the trunk-circuit at the outgo- -in g end when a connection exists therewith over which current isadapted to flow for the operation of telephonic apparatus, said re- 1 10 the presence of earth tardation-coil being qnormally disconnected operationof the relay, substantially described; 1

from "the trunk, whereby the presence of f I earth currents will notefiect the undesired 19. The combination with a trunkeline adapted to be connected with a calling telerentis adapted to flow for the operation of telephonic a paratus, whereby the undesired operation 0 the relay by. earth currents prevented, substantially as described. 20. The combination with a trunk-circuit extending between different switchboardsections, of a trunk-relay at the incoming end of said trunk energized by current over one limb only of the outgoing end, a second relay energized over one limb only of the incoming end when a connection established with a i called line, and a signal at said incoming end which will be displayed at the actuation of either of. said relays but eflaced at the actnation of both, substantially as described;

- Signed by me at Chicago, county of Cook, State of Illinois, this 26th day of July, 1902.

, FRANCIS W. DUN BAR.

Witnesses: 4 e

ROBERT LEWIS AMES, GAZELLE BEDER. 

